Turbine driven starters for starting gas turbine engines have been used for a number of years. Representative patents include U.S. Pat. Nos. 3,087,305; 3,521,505; 4,899,534; 4,926,631, all of which are assigned to the assignee of the present invention, as well as U.S. Pat. No. 4,914,906. Each of these patents depicts an air turbine starter adapted to receive motive gas flow and to produce output rotation of a rotor. The rotor is coupled by various types of gear and clutch means to an output shaft which transfers power to the gas turbine engine. By this arrangement, the motive gas flow impinging on the air turbine commences a start cycle wherein the rotating assembly within the gas turbine engine is accelerated to self-sustaining speed, this speed being the starter assist speed. Once starter assist speed has been obtained, the motive gas flow to the starter is shut off and the clutch within the starter decouples to prevent the starter from being driven by the engine.
Various clutches such as those within the above cited patents have been utilized to engage/disengage the starter unit from the engine. The U.S. Pat. No. 4,914,906 patent describes a double clutch mechanism in which the two clutches operate in parallel according to the relative speeds of the starter and the engine. The clutch mechanism utilizes a primary clutch to engage during low engine speeds and a secondary clutch to engage during high starter speeds. While the U.S. Pat. No. 4,914,906 clutch mechanism allows for engine restart at any speed, it allows ratcheting of the secondary clutch at high speeds and ratcheting of the primary clutch at low speeds. Such an arrangement requires lubrication of at least one clutch at all times after commencement of the start cycle.
Accordingly, a need exists for an air turbine starter with a non-lubricated clutch mechanism which transmits torque for speeds within a desired operating range, but completely disengages during engine overrunning. Preferably, this clutch mechanism is completely engaged prior to commencement of any starting operation, thus providing minimum backlash and minimum engagement torque.